1. Field of the Invention
In consideration of problems relating to communications, a high-stability oscillator has recently been employed in wireless devices, etc. A typical example of such oscillators is a digital temperature compensation oscillator capable of digitally compensating a variation in oscillation frequency due to temperatures. FIG. 1 is a block diagram illustrating an example of the digital temperature compensation oscillator.
2. Description of the Related Art
The temperature compensation oscillator comprises a temperature detection portion 11, an A/D conversion portion 12, a stored-data read-out portion 13, a D/A conversion portion 14 and a voltage controlled oscillator 15. An analog output from the temperature detection portion 11 is digitized by the A/D conversion portion 12 into temperature data, and the temperature data is supplied to the stored-data read-out portion 13. Compensation data corresponding to the temperature data is read out from the read-out portion 13. The read-out compensation data is converted by the D/A conversion portion 14 into analog data or compensation voltage. The compensation voltage is applied to a voltage variable capacity element (not shown) of the voltage-controlled oscillator 15. Thus, temperature-compensated oscillation frequency is obtained.
In the digital temperature compensation oscillator having the above structure, however, compensation voltages corresponding to all frequency variation components at respective temperatures are A/D converted and the resultant compensation data are directly stored in a storage portion. Consequently, an expensive large-capacity storage portion must be employed in order to high stability of frequency.
In addition, in the prior art, when a small-capacity storage portion is used, frequency/temperature characteristic is approximated in a sawtooth shape as shown in FIG. 2 and stored in the storage portion. Accordingly, the oscillation frequency/temperature characteristic varies non-continuously in a digital manner. As a result, resolution decreases and high-stability oscillation frequency cannot be obtained.
Furthermore, in the prior art, when a temperature corresponding to an intermediate address between a certain address and its adjacent address in a storage portion is detected, one of the certain address and the adjacent address is normally selected. Thus, the storage capacity determines the limit of frequency stability. Consequently, digital-wise discontinuity in frequency/ambient temperature characteristic becomes more conspicuous, and high-stability oscillation frequency cannot be obtained.